The present invention relates to a locking device of the type in which a cylinder defining a chamber that is filled with a working fluid receives a piston on a piston rod that divides the chamber into two working sections and a flow connection between the working sections is controlled by a first valve and a second valve arranged in series.
U.S. Pat. No. 5,450,933 describes and shows a locking piston/cylinder device, for example, for use with doors, in which a flow connection through the piston has two throttle points in series to allow fluid flow between two working sections, one on each side of the piston. The flow at each throttle point is controlled by a valve that has a valve body arranged in step fashion that is biased in the closing direction by a spring. A small pin on both valve bodies seals a flow connection between the two throttle points or working sections. An annular shoulder on the valve bodies provides a pressure-affected surface for the one flow direction, and the projected end face of the pin provides a pressure-affected surface for the other flow direction. The opening motion of both valves and therefore the securing force of the locking device is determined by the springs of the two valve bodies, since both valves must be open when the working medium flows. Very strict relationships are, therefore, imposed on the design of the securing forces in relation to the opening forces, which can lead to the opening forces being too great or too small. The object of the two valves connected in series lies, among other things, in that a slightly greater force is required for opening the valve and a smaller force is required for keeping the valve open, wherein one valve would be completely sufficient for one through-flow direction. If the intent is to realize this operational behavior for both flow directions, then two valves must be used. For reasons of installation space economy, the two valves are arranged in a flow connection and must, therefore, inevitably be open at the same time.